The generation of neutrophils from hematopoietic precursors and their release to the peripheral circulation are highly regulated processes that ensure the maintenance of homeostatic neutrophil levels in the blood and their rise in response to bacterial infections and other signals. G-CSF has emerged a critical regulator of granulopoiesis since mice carrying homozygous deletions of colony-stimulating factor (G-CSF) or its receptor are severely neutropenic, and dominant-negative mutations of G-CSFR have been linked to severe defects of granulopoiesis. Administration of G-CSF induces an expansion of myeloid lineage cells in the bone marrow, and promotes the release of neutrophils and hematopoietic progenitor cells from the bone marrow to the peripheral blood. Based on these properties, G-CSF is widely used to induce granulopoiesis and to mobilize hematopoietic progenitors to the peripheral blood. The biological activities of G-CSF are solely mediated by its activation of the G-CSF-receptor (R) that is expressed on myeloid lineage progenitor cells. Compelling evidence from genetic studies and other studies demonstrated that G-CSF indirectly promotes hematopoietic cell and neutrophil mobilization to the peripheral blood by modulating the activities of the chemokine SDF-1 and/or its receptor CXCR4, which are essential for the retention of hematopoietic cells to the bone marrow cavity. AMD3100, a competitive inhibitor of SDF-1 binding to its receptor, and a mutant form of SDF-1?, which induces prolonged downregulation of the CXCR4 surface receptor, promote the mobilization of neutrophils and hematopoietic cells to the peripheral blood. Osteoblasts, stromal cells and endothelial cells constitutively express SDF-1 in the bone marrow; hematopoietic cells express CXCR4. During stem cell mobilization with G-CSF, SDF-1 and CXCR4 protein levels decrease in the bone marrow. We have examined the mechanisms responsible for reduced CXCR4 expression. Initially, we found that G-CSF reduces CXCR4 expression in bone marrow Gr-1+ myeloid cells, which express G-CSFR. This is consistent with earlier observations that CXCR4 levels are reduced on neutrophils and CD34+ hematopoietic cells found in the circulation after G-CSF-induced mobilization. In additional studies, we have obtained evidence that the transcriptional repressor Gfi-1 is involved in G-CSF-induced mobilization of granulocytic lineage cells from the bone marrow to the peripheral blood. We found that in vitro and in vivo G-CSF promotes expression of Gfi-1 and down-regulates expression of CXCR4. Gfi-1 binds to DNA sequences upstream of the CXCR4 gene and represses CXCR4 expression in myeloid lineage cells. As a consequence, myeloid cell responses to the CXCR4 unique ligand SDF-1 are reduced. Thus, Gfi-1 not only regulates hematopoietic stem cell function and myeloid cell development but also likely promotes the release of granulocytic lineage cells from the bone marrow to the peripheral blood by reducing CXCR4 expression and function. In related studies, we have discovered that fibroblast growth factor-2 (FGF-2), a member of the FGF family of proteins, which play important roles in developmental morphogenic processes, reversibly reduces SDF-1 production by bone marrow stromal cells. The mechanism for such reduction was linked to post-transcriptional acceleration of SDF-1 decay. Bone marrow stromal cells express 5 FGF receptors among the 7 known FGF receptors. Blocking experiments identified FGFR1 IIIc as the receptor mediating FGF2 inhibition of SDF-1 expression in bone marrow stromal cells. In an extension of these studies, we have treated mice with FGF2 and examined its effects on bone marrow stromal cells and hematopoiesis. Systemic administration of FGF2 reversibly reduced bone marrow levels of SDF-1 and cellularity and induced immature myeloid cell mobilization, extramedullary hematopoiesis and splenomegaly. This syndrome associated with FGF2 administration to mice resembles what is normally observed in patients with clonal myeloid disorders. Reduction of intramedullary hematopoiesis and the development of myelofibrosis associated with splenic hematopoiesis are frequent complications of clonal myeloid disorders that cause severe morbidity and death. Interestingly, FGF2 levels are reported abnormally elevated in the bone marrow of patients with clonal myeloid disorders. Thus, endogenous FGF2 may represent a potential therapeutic target in clonal myeloid disorders characterized by bone marrow failure